The present invention relates to an internal combustion engine for a work tool, especially for a portable work tool, such as a motor chainsaw, etc. wherein the internal combustion engine is comprised of a cylinder having a combustion chamber and a reciprocating piston moving in the cylinder. The piston is connected by a connecting rod to the crankshaft of the engine which is rotatably mounted in a crankcase. The crankshaft has at least one bearing pin with a mounting end that is secured in a receiving bore of a crank web, and the assembled crankshaft is rotatably supported by the bearing pin in the crankcase. The connecting rod is connected to the crank pin which is mounted with its mounting end in a further receiving bore of the crank web. The longitudinal center axis of the crank pin is arranged at a stroke spacing to the parallel extending rotational axis of the crankshaft.
An internal combustion engine suitable for hand-held portable work tools is especially a single cylinder two-stroke engine which is of a simple design and provides a minimal weight in comparison to its output. Due to the size of the work tool it must be ensured that the internal combustion engine requires only a minimal mounting space so that the work tool is of a small size and can be easily manipulated.
Because of more favorable exhaust gas behavior, single cylinder four-stroke motors are also becoming more popular in hand-guided, portable work tools wherein the four-stroke motor is of a much larger size because of the intake and exhaust valves and the required valve drives. In order to be able to mount four-stroke motors in portable, hand-guided work tools, a size reduction is required. From U.S. Pat. No. 5,207,120 an assembled crankshaft for an internal combustion engine is known in which the crank pins and the bearing pins are positive-lockingly secured by press fit in the crank webs. The mounting ends of the crank pins and bearing pins have a smaller diameter than the hardened regions of its running surfaces. The resulting step in the diameter is mechanically greatly stressed and must therefore be properly sized. This results in a stroke spacing between the longitudinal center axis of the crank pin and of the rotational axis of the crankshaft that cannot be reduced.
It is therefore and object of the present invention to provide an internal combustion engine of the aforementioned kind for small-sized work tools having a constructive length in the direction of the vertical cylinder axis that is reduced as much as possible.